Simultaneous hourly measurements of lake surface water temperature (LSWT) during summer and early autumn 2000 in 29 lakes in the Swiss Alps revealed the presence of two altitudinally distinct thermal regimes. The threshold separating the low-altitude from the high-altitude regime was located at ϳ2,000 m above sea level during early summer 2000 but rose as summer progressed. Within the low-altitude regime, LSWTs are strongly related to altitude and surface air temperature. On crossing the threshold to the high-altitude regime, the LSWT lapse rate increases sharply, but the relationship of LSWT to both altitude and air temperature weakens considerably. A difference in the response of low-altitude and high-altitude mountain lakes to climatic forcing in early summer may have implications for climate change studies in which mountain lakes are employed either for paleoclimate reconstructions or as sensitive indicators of current climate change. Any long-term temporal change in the threshold altitude would imply that lakes close to the threshold may not always have been located in the same thermal regime, with consequences for paleolimnological climate reconstructions. Predictions of the effects of future climate warming on high-altitude mountain lakes may have to take into account the possibility of a concomitant rise in the threshold altitude.As the climate change debate continues, the importance of knowing how lakes respond to climatic forcing is becoming increasingly apparent. First, from a limnological perspective, an understanding of how lacustrine systems respond to climatic forcing is a necessary prerequisite to predicting their response to scenarios of future climate change. Second, from a climatological perspective, much of our knowledge of past climate changes is derived from the paleolimnological analysis of lake sediments. It is thus doubly important to establish the nature of the link between climatic forcing and lake response.The most direct impact of climatic forcing is on physical lake variables. Of these, the one that is arguably the most directly affected by climatic forcing during the biologically 1 Corresponding author (living@eawag.ch).
AcknowledgmentsWe are grateful to Carlo Casty and Oliver Heiri for their help in deploying the thermistors. Data on air temperature and relative sunshine duration were provided by the Swiss Meteorological Institute (MeteoSchweiz). The manuscript benefited considerably from the comments and suggestions of two anonymous reviewers.
